It is known that the light emission or the brightness of an LED correlates with the current flow through the LED. For brightness regulation (dimming), LEDs are therefore preferably operated in a mode in which the current flow through the LED is regulated.
In principle, it is already known to supply electric power to an LED string, which can have one or more LEDs connected in series, from a constant current source. It is likewise known to use pulse width modulation (PWM) to implement dimming, with the result that constant current regulation can be implemented in the on times of a PWM pulse train. During dimming, the duty factor of the PWM signal is then varied.
In order to provide the supply voltage of the constant current source, an actively clocked PFC (Power Factor Correction) circuit can be used, for example.
Finally, yet further requirements also need to be taken into consideration in the operation of LEDs. For example, galvanic isolation between the LED string and the supply voltage of the PFC, typically a mains voltage, is generally required.
These requirements are provided, for example, by an LED converter with a clocked constant current source, as is known from DE 10 2010 031239 A1, for example. The clocked constant current source described therein can also be in the form of a flyback converter.
LED converters are also known which can supply a variable load, i.e. a different, variable number of LEDs or LEDs of different types in the LED string. In particular for this reason, the use of flyback converters, for example, is preferred since this type of converter can be set relatively flexibly and, with such converters, it is possible for there to be an effective response to a change in the load operated by the LED converter (caused by adding or removing LEDs and/or by a change in temperature, for example).
In this case, for example, the number of LEDs can vary between 1 and 16. Thus, the LED converter needs to be capable, for example, of providing an output voltage of 3 volts for a (single) LED, for example, whereas it needs to provide an output voltage of 48 volts for, for example, 16 LEDs connected in series.
In particular when using a flyback converter, the amount of energy which can be transmitted by said flyback converter is limited, however, since the component parts, in particular the primary-side winding, cannot be enlarged to an unlimited extent.
A further problem with the flyback converter consists in that a control circuit for controlling or regulating the switch of the flyback converter is provided on the primary side. In order that the control circuit can implement the control or regulation, typically measurement signal feedback from the secondary side of the flyback converter to the control circuit takes place, wherein this feedback likewise needs to take place with galvanic isolation in order to maintain the galvanic isolation.
In order to achieve this, an optocoupler is used, for example, which makes it possible to feed back the measurement signal with galvanic isolation. The use of an optocoupler results in relatively high costs in comparison with the total costs of the circuit, however. Furthermore, the life and also the stability over time of the optocoupler are limited.
Furthermore, resonant converters have long been known, for example, from the field of ballasts for fluorescent lamps. In this field, resonant converters are used, for example, to generate a high voltage necessary for operation of a fluorescent lamp.
The resonant converter (“LLC resonant converter”) is in particular a form of DC-to-DC converter which operates with a resonant circuit for energy transmission. The resonant converter in this case converts a DC voltage into a single-phase or polyphase AC voltage and is typically operated on an approximately constant load for optimum operation. Resonant converters operate during constant operation (i.e. during operation on a constant load) at a predefined frequency working point on the resonance curve.
One disadvantage, however, consists in that in the event of a change in load owing to a variation of the LED string (different LEDs or a different number of LEDs in the series circuit of LEDs), the frequency working point on a resonance curve also shifts and the resonant converter therefore no longer operates in optimum fashion.
However, this means that not only the voltage gain, i.e. the ratio of bus voltage to output voltage, varies, but also the phase angle Θ (angle between the current IL and the voltage Vbus, as illustrated in FIG. 1) varies.
Therefore, a reactive range may result, i.e. an increase in the reactive current caused by a phase shift, in which range the efficiency of the resonant converter decreases.
Therefore, the frequency working point for the resonant converter when using 16 LEDs, for example, is very much closer to a resonance peak than when using only one LED, in which case the frequency working point is shifted considerably upwards, i.e. away from the resonance peak. Thus, the efficiency during operation with one LED is markedly reduced.
The invention therefore addresses the problem of providing an LED converter which is embodied with a resonant converter and which enables a variable and flexible operation in the case of a varying load. At the same time, signal feedback which takes place with galvanic isolation will be dispensed with.